Modern pipeline coatings are excellent and cathodic protection helps prevent small corrosion leaks from occurring as long as the coating is intact. Generally, pipeline leaks occur in older pipelines and on newer pipelines which are subject to random events such as washouts and wayward bulldozers.
To date, one of the more effective ways of detecting pipeline leaks has been by meter variance. In accordance with this technique, oil is metered into and out of the subject pipeline and the meter readings are compared. Since meters never completely agree, a problem is determining whether the meter disagreement is a leak and not one of other reasons for variance.
For most pipelines at this time, leaks are monitored by other methods which include periodic checks and other continuous monitors. Periodic checks include pressure testing, line walking, air patrol and sonic pigs. Continuous monitoring includes pressure monitoring, rate of flow monitoring, and over and short calculations. With respect to the latter, at given time intervals, input volume, output volume, and line pack are calculated and corrected for pressure and temperature. When the calculation shows a shortage, a leak is indicated. Large, computer-controlled pipelines utilize computerized over and short calculations as part of the pipeline accounting. Of course, over and short calculation is really a meter variance system with a long period between checks.
The above described meter variance is one of the more precise means for continuous leak detection. Meter variance may be conducted with computers or without. Such non-computer leak detectors are known as meter flow comparators which include a digital differential counter which continuously totals input and output volumes and calculates the difference over a moving time period. Two differential comparisons take place, one over a short period and one over a long period. Temperature and line packing effects are expected to compensate out over the long time period.
By comparison, a computer-based leak detector includes features which allow it to perform better than the simple flow comparator. These features include compensation for line packing, compensation for temperature, predictable meter factor change, improved signal transmission, better interpretation of alarms, and easy future expansion. Precise determination of line packing requires knowledge of the pressure profile along the pipeline, flow rate into and out of the pipeline, temperature profile along the pipeline, fluid gravity, viscosity, and meter factor variation with the above properties. Such a program is quite complex and its complexity is part of the problem with such devices for detecting leaks. Although the computer-based leak detector is superior to the flow comparator, there is still room for considerable improvement. The present invention is computer-based but omits many of the complexities of the prior art systems and provides a reliable and accurate system for leak detection.
The prior art includes the following articles:
V. L. Streeter and B. Wylie, "Hydraulic Transients", McGraw Hill, New York, 1967, Pages 22-62; PA1 L. Bergeron, "Water Hammer in Hydraulics and Wave Surges in Electricity", John Wiley & Sons, New York, 1961; PA1 Frank J. Berto, "Computer Aids Line-Leak Detection", The Oil & Gas Journal, Dec. 3, 1973.